Flexography is a direct rotary press method which uses relief plates made with photopolymeric materials. The plates are flexible and soft, and hence the name flexography. Such plates are inked and the print is obtained by means of directly depositing the ink on the support to be printed due to a slight pressure exerted by a printing cylinder on which the plates are positioned.
Flexography is a high speed printing process capable of printing on many types of absorbent and non-absorbent materials. Certain typical applications of flexographic printing are the making of paper and plastic bags, milk cartons, disposable cups, and the like, however by virtue of the progress today in print quality, flexographic printing is also used to print newspapers, for example, as well as bags, labels, and print on plastic, acetate film and sheets, parcel paper and for many materials used in packaging products.
A particular flexographic printing process is “digital flexography”, in which the use of digital plates is provided. The photo-polymer in such plates in origin is covered by a surface layer of material which does not allow photo-exposure, such as for example a layer of carbon or graphite. Such a layer of material is etched so as to create the negative image of the print subject. Such an etching step typically is performed through a digital laser controlled by a computer (hence the acronym CTP “computer to plate” with which the field indicates this step of the pre-printing process). The plate is then photoexposed and the material not exposed to light is eliminated by means of a washing operation actuated at a washing unit.
In particular, the washing of the plate typically is performed with solvent-based liquid which dissolves the polymer or the monomer. Alternatively, the washing liquid may be water-based. In any case, there is provided the mechanical action of brushes which act on the surface of the plate to promote the detachment of the monomer or of the polymer previously not exposed to the light.
Washing with solvent-based liquid is more common with respect to washing with water-based liquid, despite the use of solvent has various drawbacks, among which the release of volatile organic compounds (VOC), disposal difficulties and, more generally, the impact in terms of environmental safety are reported. Although water-based washing does not have these drawbacks, it requires a stronger mechanical action because unlike the solvent, water does not dissolve the polymer but only softens it. This results in an increased number of brushes and a more complicated water network for bringing the washing liquid at the brushes. In addition to this, the liquid volumes are also greater.
Typically, to wash flat plates, washing units are used provided with a chamber, inside of which there is defined a movement plane along which the plate is caused to advance. The washing brushes act on the surface of the plate opposite to the one resting on the movement plane. There are hydraulic terminals positioned in the chamber which bring the liquid at the washing brushes. In the case of water-based washing, the size of said unit become significant precisely for the above-indicated reasons, i.e. the need to have many brushes and accordingly the need for an extensive water network.
Operating stations are known for washing “sleeve” type flexographic plates, i.e. plates which are mounted on a support cylinder before undergoing the photo-exposure step. Such a cylinder carries a plate for the whole forming process thereof and is positioned in a chamber. Two washing brushes typically are installed in the latter, which brushes rotate with respect to an axis parallel to the one of the support cylinder. The washing of the “sleeve” plates occurs when solvent-based liquid is released above them by means of a water network. To this end, it has been noted that this delivery of liquid above the brushes does not allow a homogeneous bathing of the surface of the plate, while affecting the volume of liquid required and the size of the water network used.
It has been found that the stations for washing “sleeve” type plates have more contained size with respect to the ones traditionally used for washing flat plates. Moreover, also the washing times are more contained for the sleeve type plates with respect to flat plates. Therefore, the washing of a plate mounted beforehand on a support cylinder has potential advantages with respect to the washing thereof on a movement plane.
However, washing stations of sleeve type plates currently known are not operatively versatile. In particular, they do not lend themselves to washing with water-based liquid which as noted above, requires an increased number of brushes. In particular, it has been noted how in current washing machines dedicated to “sleeve” plates, the water network and the system for delivering the liquid in the chamber are entirely unsuitable for water-based washing.
Therefore, in light of the abovementioned considerations, it is the main task of the present invention to provide a washing unit of flexographic plates which allows the above-indicated drawbacks to be overcome. Within the scope of this task, it is a first object to provide a washing unit which may be used for solvent-based or water-based washing. It is another object to provide a washing unit which allows a homogeneous delivery of the liquid to the brushes and therefore a quicker and more effective washing. It is a further object of the present invention to provide a washing unit which allows a washing speed which is equal to or less than the etching speed of the plate. Not last, it is an object to provide a treatment unit which is reliable and easy to make at competitive costs.